ASSESSMENT OF SOIL TECHNOGENIC CONTAMINATION WITH HEAVY METALS DURING THE DEVELOPMENT OF THE SURGIL GAS FIELD ON THE DRIED BOTTOM OF THE ARAL SEA

ABSTRACT

The article examines the technogenic contamination of soils with heavy metals in the Surgil gas field development zone, located in the southwestern part of the dried bottom of the Aral Sea. The purpose of the study was to quantitatively assess the spatio-temporal dynamics of soil and ground contamination during the period 2006-2022 based on contact measurement data. A comprehensive approach was used, including the method of gradient indication of the degree of pollution, analysis of the polluentation radius, and calculation of the total concentration of heavy metals according to the Le Chatelier criterion. Accumulation of Pb, Cd, Ni, Zn, and Cu with pronounced spatial heterogeneity, due to the orography of the territory and hydrogeological conditions, has been established. The most intensive distribution of pollutants has been identified in the northern and northeastern directions. The contamination radius of individual elements exceeds 1 km. The values of the total concentration (q>1) indicate a high level of environmental risk. The obtained results indicate an increase in the technogenic load on the ecosystem of the dried bottom of the Aral Sea.

АННОТАЦИЯ

В статье рассматривается техногенное загрязнение почв тяжёлыми металлами в зоне разработки газового месторождения Сургиль, расположенного на юго-западе осушенного дна Аральского моря. Целью исследования являлась количественная оценка пространственно-временной динамики загрязнения почвогрунтов в период 2006–2022 гг. на основе данных контактных измерений. Использован комплексный подход, включающий метод градиентной индикации степени загрязнения, анализ радиуса действия поллютантов и расчёт суммарной концентрации тяжёлых металлов по критерию Ле Шателье. Установлено накопление Pb, Cd, Ni, Zn и Cu с выраженной пространственной неоднородностью, обусловленной орографией территории и гидрогеологическими условиями. Наиболее интенсивное распространение загрязняющих веществ выявлено в северном и северо-восточном направлениях. Радиус загрязнения отдельных элементов превышает 1 км. Значения суммарной концентрации (q>1) указывают на высокий уровень экологического риска. Полученные результаты свидетельствуют о нарастании техногенной нагрузки на экосистему осушенного дна Аральского моря.

Keywords: dried bottom of the Aral Sea, technogenic soil contamination, heavy metals, gas field development, Surgil gas field, spatial–temporal dynamics, gradient indication method, environmental risk.

Ключевые слова: осушенное дно Аральского моря, техногенное загрязнение почв, тяжёлые металлы, газовое месторождение Сургиль, пространственная динамика, градиентная индикация, экологический риск.

Introduction. The development of hydrocarbon deposits is associated with a heavy burden on the natural environment. Negative impacts occur both within the territories of the deposits themselves and in the nearest settlements. There is pollution of soils, surface water bodies and watercourses, groundwater, destruction of soil cover and deterioration of ecological conditions for biota [1].

Gas fields generate a significant amount of formation water, which, after processing, is discharged in the immediate vicinity of the gas fields [2]. The wastewater contains many hazardous elements, including dissolved and dispersed hydrocarbon components, chemical additives, formation minerals, heavy metals, naturally occurring radioactive minerals, dissolved substances, ions, and so on [3, 4].

Publications on the environmental impact of gas fields pay particular attention to the contamination of soil and groundwater with heavy metals [2, 5, 6]. Heavy metal contamination of soil has become a serious problem in recent years. These elements can accumulate in plants and animals, ultimately entering the human food chain. The quality of water, soil, and water systems is heavily dependent on the industrial sector. Heavy metals are the most important soil contaminants originating from geogenic and anthropogenic sources, as they are persistently harmful and pose an additional health risk that must be regulated in our environment. These toxins can spread to other parts of the environment through air, transportation, and gravity [7, 8, 9]. However, some heavy metals are associated with soils, which are considered the primary repository when released into the environment. Furthermore, heavy metal contamination of soil is exacerbated by the fact that chemical elements and compounds can be transported through geochemical processes such as dissolution and adsorption. This problem is caused by the increased difficulty of controlling various pollutants that accumulate in the soil and the greater threat of deterioration in agricultural production caused by the impact of other sources of pollutants.

Heavy metals that enter the air with gas and dust emissions, through sedimentation and atmospheric precipitation, settle on the terrain, accumulate, and contaminate the soil [10]. It is well known that heavy metals are not subject to chemical decomposition and must be physically removed or converted into non-toxic compounds. In global environmental monitoring practice, many methods and techniques have been developed to reduce the content of heavy metals in soils. Approaches based on the use of priority groups of microorganisms, algae, plants, and animals have been developed [11, 12]. Unfortunately, this method is not applicable in conditions of excessive salinity of the dried bottom of the Aral Sea soils.

In our opinion, the most promising method is phytoremediation, which is a set of technologies that use various plants to localize, destroy, or extract pollutants. Phytoremediation is a new technology that uses various plants to decompose, extract, localize, or immobilize pollutants from soil and water [13].

In the Aral Sea region, the problem of environmental pollution with heavy metals arose in connection with the discovery of the West Aral natural gas field with preliminary reserves estimated at 11 billion cubic meters and the Surgil field, whose reserves amount to about 120 billion cubic meters of natural gas. In the near future, there are plans to build a 110-kilometer pipeline to deliver natural gas from the developed Surgil field to the Kungrad soda plant [14, 15, 16], which is fraught with additional environmental stress in the form of destruction of soil and vegetation cover.  Environmental safety control of such large-scale activities is carried out in accordance with the Law "On Nature Protection" (1992) and the Regulation "On State Environmental Expertise," approved by Decree No. 949 of the Cabinet of Ministers of the Republic of Uzbekistan on November 22, 2018. As a result of monitoring activities at the above-mentioned deposits by the State Committee for Ecology and Environmental Protection of the Republic of Karakalpakstan and scientific research conducted within the framework of the fundamental research program of the Academy of Sciences of the Republic of Uzbekistan [17], a sufficient amount of factual data on the state of the natural environment in this region has been accumulated. However, the processing of the information obtained consisted mainly of comparing data on pollutants with their maximum permissible concentrations (MPC).

It should be noted that publications (including foreign ones) on this subject lack analysis of information to identify patterns and trends in the dynamics of the environment. At the same time, anthropogenic soil pollution, like the composition of substances in general, is highly dynamic both in time and space. In particular, the dynamics of long-term anthropogenic pollution are determined by the following processes [14, 15, 16, 18, 19, 20]:

1) the accumulation of pollutants;

2) the destruction of complex organic substances and their conversion into simple inorganic compounds, as well as chemical reactions between soil components;

3) the influence of soil microflora;

4) the influence of terrestrial biota through accumulation in plant and animal tissues;

5) diffuse, capillary-convective, and infiltration processes in the soil;

6) the environment's ability to self-clean;

7) wind erosion of contaminated surface soil layers.

Changes in the orography caused by the drying up of the sea give the long-term anthropogenic impact of pollutants on the dried bottom of the Aral Sea a special spatial dynamism. Each of the above processes deserves separate in-depth study to determine their contribution to the dynamics of soil composition near industrial facilities. To our knowledge, such studies have not been conducted in the Surgil gas field development region.

 This work addresses a more modest task: without delving into the mechanism of these processes and relying solely on contact measurement data (analysis of soil samples for heavy metal content), to obtain a quantitative assessment of the spatial and temporal dynamics of anthropogenic contamination of soils with heavy metals in the vicinity of a single well (formation water disposal reservoirs) Surgil 54 in the period 2006-2022.    

Data. The background indicators of soil composition are considered to be the data (Draft Environmental Impact Statement, 2012) for 2006, immediately preceding the start of gas field development at the exploration and design stage. To assess the condition of the soil at the first stage (2011-2012) of the activities of the joint venture UzKorGasChemical LLC, field research materials were used, as presented in the works [14, 16]. The data for 2022 obtained by the author¹ during field studies in the same area as the data for 2011-2022 are analyzed for the first time. The data include the results of soil sample analysis, formation water discharge volumes, and soil mechanical composition.

The study area (black dot) is located in the southwestern part of the Arkhangelsk embankment (Fig. 1), the highest points of which run along its center (dotted line).

Figure 1. The drained bottom of the Aral Sea in 2006 (left) and 2022 (right)

The general layout of the Surgil gas field wells and soil sampling points is shown in Fig. 1. The studies were conducted on radial segments in the north, south, northeast-east, and south-southwest directions at distances of 100 m, 500 m, and 1000 m from the Ustyurt Gas Chemical Complex. The choice of these directions was determined by the macro- and micro-relief of the terrain, hydrogeological conditions, mechanical composition of the soils, as well as wind conditions, which cause the aeolian transport of pollutants with a predominance of northern and northeast directions.

Figure 2. Sampling points near the Surgil deposit

Methods. The study used a comprehensive approach combining quantitative and monitoring methods. Quantitative methods include a method developed by the authors for gradient indication of the degree of pollution based on spatial dynamics analysis, assessment of the total concentration of pollutants, and comparative statistical analysis of anthropogenic soil pollution in different years. Monitoring methods consist of soil sampling and subsequent analysis.

The method of gradient indication of the degree of contamination by a particular ingredient, based on the analysis of spatial dynamics, consists in calculating the average gradient  of the relative value  (exceeding the background concentration value) in the radial direction from the source:

where  is the average background concentration of pollutants in the k-th direction,  is the concentration at the i-th sampling point.

The following interpretation of  values based on the impact of industrial facilities (IF) is logical:

–  – on average, the IE does not affect the content of this ingredient in the soil;

–  – insignificant increase in P with distance from the source (the source is located above the end point of the route, which causes the substance to flow away from the starting point and accumulate at subsequent points along the route);

–  – insignificant decrease in the ingredient in the soil with distance from the source;

–  – significant decrease in this ingredient in the soil with distance from the source (the source is located below the end point of the route, which causes heavy metals to accumulate near the source).

Since environmental pollution is an undesirable impact caused by the introduction of pollutants into the environment, the qualitative impact of an industrial facility is determined by the degree of harmfulness or usefulness of a given soil ingredient.

Many pollutants associated with gas production technologies have a cumulative effect. In this study, for the first time, a normative characteristic such as the total concentration of pollutants [21] is used to assess the ecological state of the soil in the southwestern of the drained bottom of the Aral Sea:

where is the concentration of a substance in a group.

According to Le Chatelier's law [21], this value must be less than or, in extreme cases, equal to 1 (q≤1), otherwise the pollution by this group of substances is considered excessively high.

Soil samples were taken after evaporation of drilling fluids from active waste at depths of 20 cm and 1 m. Analytical studies to determine pollutants in soils were carried out in accordance with the НАСН methodology using appropriate laboratory equipment (Spectrophotometer DR/2000, absorption spectrophotometer, Kvant-9M).

Analytical work was carried out by the State Specialized Inspectorate for Analytical Control (SIAC) of the State Committee for Nature Protection of the Republic of Karakalpakstan (Jollibekov M.) and the State Specialized Inspectorate for Analytical Control (ANIDI) of the State Committee for Nature Protection of the Republic of Uzbekistan (Mirrakhimov M.M.).

Discussion of results.  The data from the analysis of soil samples for heavy metal content in the vicinity of the Surgil 54 well in 2006, 2012, and 2022 are shown in Tables 1-3 with the following designations: N, NE, S, SW – directions of the routes.

Table 1.

Pb, Cd, Ni, Zn, Cu content in soil in 2006

Table 2.

Pb, Cd, Ni, Zn, Cu content in soil in 2011.

Table 3.

Pb, Cd, Ni, Zn, Cu content in soil in 2022

The variation in heavy metal content in the soil along the routes is explained by the rugged terrain, which is the former bottom of the Aral Sea (Arkhangelsk embankment). The south and southeast have rising terrain, while the north and northeast have terrain that slopes down from the source. As is known from soil physics, the terrain is of great importance for horizontal infiltration, and since the agent of heavy metals in our case is water (discharge of drilling fluids), the flow in the northern direction is greater than the flow in the southern direction. Therefore, the spread and accumulation of heavy metals is greater and faster in the north and northeast directions.

The spatial dynamics of heavy metal concentrations in different directions can be clearly and vividly reflected in concentration field graphs (Fig. 3).

Figure 3. Spatial dynamics of lead concentration in 2011 and 2022

The heterogeneity of the terrain's orography causes the formation of local concentration maxima  in lowlands and local minima  on elevated terrain (Fig. 3).

The singular dynamics of the average heavy metal content in the soil for 2006-2022, reflected in the histogram, clearly indicates the progressive nature of technogenic pollution. (Fig. 4).

Figure 4. Singular dynamics for 2006-2022 of the average content of heavy metals in the soil.

The dynamics of the average heavy metal content in the soil is expressed by positive trends (Table 4).

Table 4.

Trends in the dynamics of the average content of heavy metals in the soil

The gradient indication method was used to obtain estimates of soil contamination for 2011 and 2022 (Table 5).

Table 5.

G values for Pb, Cd, Ni, Zn, Cu

Overall, calculations using our proposed gradient indication method showed that gas fields have a negligible impact on heavy metal content.

As the calculations showed, the radius of contamination in 2023 is more than 1 km for all heavy metals and for all directions (Table 6).

Table 6.

Radius of action of heavy metals for all directions

Calculation of the total concentration at a distance of 100 meters from the source of pollution showed that, given the summation effect for the specified metals, soil contamination is significant, as it is greater than 1 in all directions: N – 2.816, NE – 3.823, S – 3.213, SW – 2.712.

Table 7.

Total concentration values for heavy metals

The table shows a significant increase in the total concentration of heavy metals in the soil over the period 2011-2022.

Conclusion. Based on the study of the impact of gas field development on the drained bottom of the Aral Sea on the content of heavy metals in soil, the following conclusions have been made.

In general, the process of soil contamination with heavy metals can be represented as the anthropogenic transfer of substances from deep strata to surface layers and their spread from the source by horizontal infiltration. The subsequent aeolian redistribution of heavy metals from the zone of influence of gas fields to nearby territories  should also be taken into account. 

The heterogeneity of the drained bottom's orography causes local maxima in heavy metal concentrations due to accumulation in lowlands and local minima on relief elevations. This circumstance, combined with a number of noises created by the processes listed in the introductory part of the article, allows only an aggregated assessment of soil contamination during the development of gas fields.   Nevertheless, the use of a comprehensive approach made it possible to obtain the following results, which have elements of scientific novelty.

The proposed method of gradient indication of the degree of environmental pollution by industrial facilities made it possible to determine the zone of influence of the Surgil 54 field on the content of heavy metals in soils, taking into account the orography of the studied area. On average, the heavy metal content at the surveyed points does not exceed the maximum permissible concentration (MPC), but in low-lying areas, the accumulation of heavy metals can lead to MPC exceedances.

Many pollutants associated with gas production technologies have a cumulative effect. In this work, for the first time, a normative characteristic such as total concentration is used to assess the ecological state of the soil in the southwestern part of the drained bottom of the Aral Sea. This method, in conjunction with Le Chatelier's criterion, made it possible to determine that heavy metal contamination of soils during the development of the Surgil gas fields exceeded the permissible level.

The dynamics of technogenic soil contamination with heavy metals in the period 2011-2023 were traced and quantitatively assessed using analytical expressions. The trends obtained indicate high rates of increase in the content of heavy metals in the soils of the drained bottom of the Aral Sea under the influence of man-made pollution.

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